This invention relates to a biocompatible titanium base alloy characterized by high strength, low modulus and ductility, and to a method for the preparation of said alloy. The alloy of the invention is particularly suitable for the manufacture of prostheses and the invention also is concerned with a prosthesis made from the alloy.
Titanium base alloys for a variety of structural applications are known in the art and there are numerous patent and literature references disclosing a wide range of alloying elements which are used to provide alloys having desired characteristics, such as increased tensile strength and ductility. Generally, titanium and its alloys may exist in one or a mixture of two basic crystalline structures: the alpha phase, which is a hexagonal close-packed (HCP) structure, and the beta phase which is a body-centered cubic (BCC) structure. The transition temperature from the alpha to the beta phase is about 882.degree. C. for pure titanium. Elements which promote higher transformation temperatures are known as alpha-stabilizers. Examples of alpha stabilizers are aluminum and lanthanum. Elements which promote lower transformation temperatures are known as beta-stabilizers. Beta stabilizers are classified in two groups: the isomorphous beta stabilizers, exemplified by molybdenum, niobium, tantalum, vanadium and zirconium; and the eutectoid beta stabilizers, exemplified by cobalt, chromium, iron, manganese and nickel. Thus, depending upon the type and amount of alloying elements, there are three general classes of titanium base alloy: alpha, alpha-beta and beta.
An example of a high strength titanium base alloy containing the beta stabilizers vanadium and iron and the alpha stabilizer aluminum is disclosed in U.S. Pat. No. 3,802,877. However, the biocompatibility of this alloy may be compromised because of the presence of vanadium, which should be avoided in an alloy used to fabricate an implant.
Bone implants made from titanium or titanium-containing alloys are known in the art. Implants, such as plates and screws, made from pure titanium were used in 1951 for the fixation of bone fractures when it was found by Jergesen and Leventhal that these implants exhibited good tissue tolerance. See Laing, P. G. "Clinical Experience with Prosthetic Materials," ASTM
Special Technical Publication 684 (1979), pp. 203-4. However, although pure titanium has excellent corrosion resistance and tissue tolerance, its relative low strength, when compared to stainless steel, and unfavorable wear properties, limited its use for general bone implants.
In the 1970s pure titanium for surgical implants was replaced by an alloy containing aluminum and vanadium (Ti-6Al-4V) for the manufacture of high strength femoral prostheses. However, although no toxic reaction was reported in patients, the known toxicity of vanadium and the association of aluminum with various neurological disorders has raised considerable doubt about the safety of this alloy.
U.S. Pat. No. 4,040,129 discloses an implant for bone surgery and for dental therapeutics containing defined critical amounts of titanium and/or zirconium and other selected metallic elements including niobium, tantalum, chromium, molybdenum and aluminum. Alloying elements of questionable biocompatibility, such as vanadium, are specifically excluded.
In 1980 a Ti-5Al-2.5Fe alloy was disclosed for surgical implant application and in 1985 a Ti-6Al-7Nb alloy was disclosed for the manufacture of various types of femoral component stem. Each of these alloys contained a relatively high proportion of the suspect alloying element aluminum.
A biocompatible titanium base alloy suitable for bone implants should meet at least the following requirements:
1. Potentially toxic elements, such as vanadium, copper and tin, should be avoided completely.
2. Elements which may have potential toxicological problems, such as chromium, nickel and aluminum should be used only in minimal, acceptable amounts.
3. The alloy should have high corrosion resistance.
4. The alloy should have at least the following desired mechanical properties: flow modulus, high strength and good smooth and notched fatigue strength.
5. The alloy should have good workability and ductility.
It has now been found that a biocompatible alloy meeting the desired requirements and, in particular, having a combination of high strength and low modulus desirable for orthopaedics but not possessed by any alloy disclosed in the prior art, may be produced, preferably by double plasma melting, from a carefully balanced formulation of beta stabilizers, alpha stabilizers and titanium.
Thus commonly assigned U.S. Pat. No. 4,857,269 provides a high strength, low modulus, ductile, biocompatible titanium base alloy consisting essentially of the following alloying components:
an amount up to 24% by weight of at least one isomorphous beta stabilizer selected from the group consisting of molybdenum, tantalum, niobium and zirconium, provided that molybdenum, when present, is in an amount of at least 10% by weight, and when molybdenum is present with zirconium the molybdenum is an amount of 10 to 13% by weight and the zirconium is in an amount of 5 to 7% by weight; PA1 an amount up to 3.0% by weight of at least one eutectoid beta stabilizer selected from the group consisting of iron, manganese, chromium, cobalt and nickel; PA1 optionally an amount up to 3.0% by weight of at least one metallic alpha stabilizer selected from the group consisting of aluminum and lanthanum; PA1 and the balance titanium, apart from incidental impurities, but not exceeding by weight, among the non-metallic alpha stabilizers, 0.05% carbon, 0.30% oxygen and 0.02%, nitrogen, and not exceeding 0.02% of the eutectoid former hydrogen; the proportion of each of the alloying components being balanced to provide an alloy having a modulus of elasticity not exceeding 100 GPa. PA1 up to 24% by weight of at least one isomorphous beta stabilizer selected from the group consisting of molybdenum, tantalum, niobium and zirconium; PA1 up to 3.0% by weight of at least one eutectoid beta stabilizer selected from the group consisting of iron, manganese, chromium, cobalt and nickel; PA1 optionally up to 3.0% by weight of at least one metallic alpha stabilizer selected from the group consisting of aluminum and lanthanum; PA1 and the balance titanium; PA1 introducing the resulting blended feedstock into a plasma arc furnace wherein the blend is melted to form a homogenous melt, allowing the melt to cool and solidify, vacuum arc remelting the resulting solid to assure that the hydrogen content does not exceed 0.02% by weight and thermomechanically processing the resulting solid at a temperature within the range of 710.degree. to 1038.degree. C. to provide an alloy having a modulus elasticity not exceeding 100 GPa.
A preferred embodiment of the above alloy is an alloy having a modulus of elasticity of 66.9 to 100 GPa; a 0.2% offset yield strength of 925 to 1221 MPa; a rotating beam fatigue strength of 483 to 621 MPa at 107 cycles and of 345 to 380 MPa at a stress concentration factor, K.sub.t, of 1.6; and a tensile elongation of at least 10%.